The First and Ten line

I have always wondered how this was done -- excellent article with extra geeky goodness. From the IEEE's Today's Engineer:

The Making of Football's Yellow First-and-Ten Line
What would televised football be without the yellow �First and Ten� line? This graphic enhancement provides the viewer at home with an immediate visual appreciation of where the offense has to take the ball to make another first-down. For the spectators in the football stadium watching the game, there is no yellow line on the field. No matter where it�s placed, for the television audience, the yellow line appears to be an integral part of the playing field like any of the white yardage lines. When players fall on the yellow line their bodies cover it. Today, football fans at home rarely question how this yellow line appears on their television screens. To a whole generation of young television viewers, the yellow �First-and-Ten� line is as natural to the game as the green color of the football field.

In 1997, when ESPN first aired the First-and-Ten Yellow Line, amazement and wonder were the universal reactions to this new broadcasting technology. Sports journalists were baffled. How was this line put on the field? All sorts of fanciful speculation filled the air after the first broadcast of the First-and-Ten Yellow line. �Is there a guy running out there with a vacuum and chalk?� �Could it be done with laser beams?� The actual details of the innovation were as incredible as the speculation. Sophisticated modeling based on precise measurements, ingenious real-time image processing, and a truck load of workstations made the First-and-Ten Yellow Line look as natural to the game as the turf itself. Behind it all was a small start-up company whose technical team was composed of an aeronautical engineer, mathematician, broadcast engineer, software engineer and a couple of electrical engineers. The new company was Sportvision and its president was an IEEE member.

Basically Chroma key on steroids but some interesting problems to deal with -- the early high-range zoom lenses would have a bit of distortion (10% pincushion) and to overlay a perfectly straight CGI yellow line on the distorted image of the chalk lines on the field would be visually jarring. They pre-distort the lines. Needless to say, this has to be calibrated for each camera location. The end result is a visually simple and informative but technologically very complex. A great hack!